1. Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus including the same and is suitable for, for example, image pickup apparatuses including image pickup devices, such as a digital still camera, a video camera, a monitoring camera, and a broadcast camera, or image pickup apparatuses such as a silver-halide-film camera.
2. Description of the Related Art
Recent image pickup apparatuses including solid-state image pickup devices, such as a digital still camera and a video camera, have improved functions while the total sizes thereof are small. Zoom lenses intended for such apparatuses are desired to have short total lens lengths and high zoom ratios. Moreover, the zoom lenses are desired to be lightweight and to cost low.
A positive-lead zoom lens is an exemplary zoom lens having a short total lens length and a high zoom ratio and includes, in order from an object side to an image side thereof, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power, and a rear lens group including one or more lens units. Another exemplary zoom lens includes lens elements that are made of materials other than glass, such as resin materials, so that the weight and the cost of the zoom lens are reduced.
A positive-lead zoom lens is disclosed by Japanese Patent Laid-Open No. 2010-102096 in which a second lens unit includes a resin lens element having an aspherical surface and provided at the extreme end of the second lens unit on the image side. Another zoom lens is disclosed by Japanese Patent Laid-Open No. 2006-171615 in which a second lens unit includes one resin lens element having a positive refractive power and one resin lens element having a negative refractive power, and a fourth lens unit includes one resin lens element having a positive refractive power and one resin lens element having a negative refractive power.
In general, the variation in the refractive index of a resin lens element due to changes in the temperature is greater than that of a glass lens element. The effect caused by the variation in the shape of the resin lens element due to expansion or contraction is also greater than that of the glass lens element. Hence, variations in the focus, spherical aberration, and the like due to changes in the temperature are significant, leading to possible deterioration of optical performance. In a positive-lead zoom lens including a relatively small number of lens elements, the refractive power on each of lens surfaces is high. Therefore, the deterioration of optical performance due to changes in the temperature appears clearly at every zooming position.
To obtain a small-size, high-zoom-ratio zoom lens that includes resin lens elements and exhibits high optical performance at every zooming position, it is important to provide resin lens elements in a plurality of lens units and to appropriately set the refractive powers of the lens elements and the positions of the resin lens elements.